1. Field of the Invention
The present invention relates to a power supply control system for an electrically heated catalytic converter which controls an electric supply to an electrically heated catalytic converter.
2. Description of the Related Art
An electrically heated catalytic converter (hereinafter referred to as "an EHC") disposed in an exhaust gas passage of an internal combustion engine is known in the art. The electrically heated catalytic converter is used for raising the temperature of the catalysts in the converter to their activating temperature in a short time after the engine has started.
When an internal combustion engine starts, since the amount of fuel supplied to the engine is increased to facilitate the engine starting, the amount of HC and CO in the exhaust gas increases. However, by supplying electricity to the EHC, the temperature of the converter reaches the activating temperature in a short time after the engine has started. Therefore, the converter starts its catalytic action for purifying HC and CO in the exhaust gas immediately after the engine has started, whereby the emission of HC and CO to the atmosphere after the start of the engine is reduced.
An electrically heated catalytic converter of this type is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 4-279718.
The '718 publication discloses an EHC in which electricity is supplied from a battery to the EHC. The supply of electricity from the battery is continued for a period sufficient for raising the temperature of catalysts in the EHC to the activating temperature.
However, it is necessary to supply a relatively large electric power to the EHC for heating the catalysts in a short time. Therefore, if electricity is supplied from the battery, as in the system of the '718 publication, the load of the battery becomes large and damage to the battery may occur.
SAE paper No. 941042 (published in February, 1994) discloses another EHC system in which electricity is supplied to an EHC directly from a generator when the engine starts. Namely, the EHC system in the '042 paper is provided with a changeover switch which connects either of the battery and the EHC selectively to the generator. In a normal operation of the engine, the changeover switch connects the battery to the generator and charges the battery. However, the changeover switch disconnects the battery from generator and connects the EHC to the generator when the engine starts in order to supply electricity to the EHC directly from the generator.
In the EHC system of the '042 paper, since the battery is disconnected from the generator when electricity is supplied to the EHC, it becomes possible to supply a large electric power to the EHC by raising an output voltage of the generator to a value higher than a normal output voltage for charging the battery. Therefore, the EHC can be activated in a short time without imposing a large load on the battery. In the EHC system of the '042 paper, the EHC is disconnected from the generator, and the battery is connected to the generator after the temperature of the EHC reaches the activating temperature. Thus, the battery is charged by the generator in a normal operation after the EHC has activated.
However, since the exhaust gas temperature of an engine is low immediately after the engine starts, sometimes the EHC is cooled by the cold exhaust gas. Therefore, if the supply of electricity to the EHC is stopped after the EHC reaches the activating temperature, the temperature of the EHC again becomes lower than the activating temperature and the activity of the catalysts is lost. Thus, it is preferable to continue to supply electricity to the EHC even after the EHC has been activated in order to maintain the temperature of the EHC at a value higher than the activating temperature.
In the EHC system of the '042 paper, however, the battery is disconnected from the generator when electricity is supplied to the EHC. Therefore, since only the battery supplies electricity to all other electric loads of the engine when electricity is supplied to the EHC, an excessive discharge of the battery may occur if electricity is supplied to the EHC for a long period. Therefore, in the EHC system of the '042 paper, it is not possible to continue the supply of electricity to the EHC to maintain the catalytic activity of the EHC after it is activated.
On the other hand, since the battery in the EHC system of the '718 publication is always connected to the generator, it might be possible to continue to supply electricity to the EHC for maintaining the catalytic activity of the EHC even after the EHC reaches the activating temperature. However, since the battery in the '718 publication also supplies a large electric power for activating the EHC, the load of the battery becomes excessively large, and damage to the battery may occur if the battery continues to supply electricity to the EHC even after it is activated.